The Chronobiology Core is guided by the overriding hypothesis that the onset, course and outcome of geriatric mood disorders are associated with various state and trait variables that can be measured by some of the same techniques and methodologies used to examine sleep and biological rhythms in healthy subjects. By assessing these variables prior to and during the course of treatment, and relating them to clinical measures, it is hypothesized that a better understanding can be gained as possible mechanisms of action of various treatment modalities, as well as potential physiological predictors of susceptibility to and recovery from late-life depression. The Core will carry out the scientific mandate of the nature CRC by testing specific hypotheses concerning outcomes, mechanisms, and treatment of geriatric affective disorders. Outcomes. Physiological measures, including waking quantitative EEG (qEEG) and 24-hour sex hormone levels, will be investigated by our Core to determine if they can predict the occurrence of, as well as the time to clinical recovery. Both baseline and longitudinal assessments will be combined with corresponding data from other CRC Cores to evaluate whether there are qEEG or sex hormone correlates of outcomes such as dementia, disability, chronicity, recurrence relapse, or mortality. Mechanisms. The Chronobiology Core will characterize age-related changes in circadian rhythmicity in healthy older individuals. These efforts will lay the groundwork for elucidating putative mechanisms involved in increasing the vulnerability to geriatric mood disorders. Treatment. By continuing to test hypothesis concerning the chronobiological effects of electroconvulsive therapy, and relating these effects to treatment efficacy, the Chronobiology Core seeks to further the understanding of how chronobiological principles may mediate this treatment of geriatric depression, and perhaps outline other means by which chronobiological principles may enhance treatments of geriatric depression. Measurement of qEEG, sex hormone levels, and circadian rhythm variables add a unique scientific dimension to this CRC. The integration of these data with services provided by the Pathophysiology, Psychosocial and Clinical Psychopharmacology Cores is expected to generate additional testable concepts to facilitate our understanding of the etiology and treatment of geriatric mood disorders.